The reaction of oxygen with hydrometallurgical process liquids, the hydrogenation of unsaturated organic liquids, and the treatment of sewage with oxygen by the activated sludge process all rely on some means for bringing gases and liquids into contact with each other, preferably in a manner which will utilize the gas efficiently. One particularly successful process utilizes a draft tube containing a helical impeller with a baffled inlet at the head of the tube. While the gas/liquid contact area, which results from the utilization of this process, is commercially acceptable, it is rather apparent that a significant increase in this parameter could be singularly advantageous in that it decreases the amount of usually expensive gas, e.g., oxygen or hydrogen, required to achieve the same result. It is not surprising, then, that the art is continuously striving to boost gas utilization efficiencies, and concomitantly, lower gas costs.